The present invention relates to an infinitely variable cone pulley transmission including two pairs of cone pulley discs disposed on the driving shaft and on the driven shaft, respectively, and a transmission means rotating therebetween, with the cone pulley discs being connected with the shaft supporting them so as to rotate together with them. On each shaft one cone pulley disc is also axially fixed while the associated, second cone pulley disc is configured as an axially displaceable component of a cylinder-piston assembly that is fixed to the shaft. Through an inlet, each cylinder-piston assembly is provided with pressure medium that is measured out by a four-sided sliding control valve for setting and maintaining the transmission ratio of the gears. The four-sided sliding control valve is equipped with an outlet for excess pressure medium quantities in which there is disposed a possibly adjustable throttle. By way of an adjustment member, the four-sided sliding control valve is in communication, for example, with one of the displaceable cone pulley discs.
Such a so-called fully hydraulic cone pulley transmission is disclosed, for example, in German Unexamined Published Patent Application 3,028,490 which corresponds to U.S. Pat. No. 4,439,170.
In cone pulley transmissions of this type it is a known phenomenon that the required axial pressure forces are of different magnitudes at the drive shaft and at the driven shaft, respectively. The ratio of the pressure forces is thus not equal to one.
When dimensioning a cone pulley transmission, it is assumed initially that the sliding through of the transmission means is prevented on the driven side in all operating states. This results in the minimum pressure of the hydraulic system.
During operation, the transmission means then exerts a pulling force from the driven side to the driving side and this force builds up a corresponding spreading force between the cone pulley discs. This spreading force, in turn, must be supported by a hydraulically generated axial force. The axial force on the driving side is here, on the average, about 25 to 30% greater than on the driven side.
With the prior art embodiment in which the cylinder-piston assemblies are the same on the driving side and on the driven side, this higher pressure force is ensured in that the four-sided sliding control valve is displaced in such a manner that the pressure generated by the pressure medium supply is distributed accordingly to the driving side and to the driven side so that a greater pressure exists at the drive shaft than at the driven shaft.
This configuration has the drawback that the pump responsible for supplying the hydraulic cone pulley transmission with pressure medium must be able under any circumstances to furnish this higher pressure to the driving side. On the driven side, this pressure made available by the pump is throttled down to the lower pressure sufficient here.
The power requirement of the pump has a direct relationship to this pressure. This power requirement in turn is an important factor for the overall efficiency of the type of transmission discussed here which has a decisive influence on fuel consumption, particularly, for example, in connection with motor vehicle transmissions operated under partial load.
German Unexamined Published Patent Application 2,518,496 discloses, for a different type of transmission without four-sided sliding control valve, the reduction of the pressure to be furnished by the pump in that the higher axial force required on the driving side is generated in that the pressure chargeable surface of the associated cylinder-piston assembly associated with the drive shaft is larger than the surface on the driven side. Thus it is possible to operate on the driving side also with the lower pressure required on the driven side.
The drawback of the possible transfer of these unequal diameter conditions to a transmission of the above-mentioned type would be that the four-sided sliding control valve which measures the pressure medium out for the cylinder-piston assemblies would be disposed approximately in its center position. A minimum curve regarding the pressure by way of the control piston path is connected with this center position. Around this center position, the equilibrium behavior of the sliding control valve is almost indifferent. Moreover, relatively long control piston paths would be required to generate a constant transmission ratio. The equilibrium pressure could here be generated, depending on the transmission position, in the cylinder on the driving side as well as in the cylinder on the driven side since the ratio of the cylinder surfaces can only be an average with respect to the pressure conditions that change by way of the transmission ratio adjustment range of the transmission. This would cause instabilities in the control.
Another drawback of such a transmission would be that, if the diameter of the cylinder-piston assembly were increased according to the greater clamping force on one side, the set transmission ratio could be maintained only while for a change in the transmission ratio, particularly if it is to be effected quickly, a further, not insignificant counterforce would have to be generated in addition.
For these reasons such a transmission is not advisable.